Normally this means that your supplied password to access the database is not the correct one.

There is a difference between the admin user of your odoo environment (admin user) and the admin user of the database server (main password or database odoo password) . if you don't remember your initial password (created when you installed odoo), or you made a type in the during the intial installation, which prevent you now to backup the database. you can edit the odoo-server.conf (or openerp-server.conf) and include a new password.